1. Field of the Invention
The invention relates to metal layers used in electronic devices, and more particularly, to metal layers formed on substrate. The invention is especially advantageous when applied to manufacturing processes for thin film transistors (TFTs) which are used in, for example, liquid crystal panels.
2. Description of the Related Art
A technique for forming a tri-layer metal structure is described in U.S. Pat. No. 5,428,250 to Ikeda et al. The tri-layer metal structure is formed on a glass substrate. The first layer is a Ta-M-N film, the second layer is a Ta film and the third (top) layer is a Ta-M-N film, where M is at least one atom selected from the group consisting of Molybdenum, Niobium, and Tungsten.
U.S. Pat. No. 5,153,754 to Whetten described a tri-layer metal structure formed on an LCD substrate where the first layer is a titanium (Ti) film, the second layer is a molybdenum (Mo) or aluminum (Al) film, and the third (top) layer is a titanium (Ti) film. In addition, column 6, lines 56-70 describe a process to taper etch the tri-layer metal structure. When the second layer is a molybdenum film, the tri-layer structure is formed by wet etching the titanium first layer with fluoroboric acid (HBF.sub.4), wet etching the molybdenum second layer with PAWN (phosphoric acid, acetic acid, water and nitric acid), and dry etching the titanium third layer in a plasma barrel etcher with an atmosphere of CF.sub.4 and O.sub.2 (or SF.sub.6 and O.sub.2). When the second layer is an aluminum film, the tri-layer structure is formed in a single etch step by an RIE etch of BCl.sub.3, CCl.sub.4 and O.sub.2.
U.S. Pat. No. 5,464,500 to Tsujimura et al. describes a tri-layer metal structure formed on a glass substrate. A silicon oxide layer is formed on the glass substrate. The first metal layer of Aluminum (Al) is formed on the silicon oxide layer. The second metal layer of aluminum oxide is formed on the first metal layer. The third metal layer of molybdenum is formed in the aluminum oxide layer. Beginning at column 3, line 60, a process for taper etching the tri-metal layer is described. As a result, the cross section of the first metal layer of aluminum is formed with a taper angle.
U.S. Pat. No. 4,824,803 to Us et al. describes a tri-layer metal structure formed on a glass layer wherein the first metal layer is a titanium (Ti) film, the second metal layer is an Aluminum (Al) film, and the third metal layer is a titanium film. As described beginning at column 2, line 45, the tri-metal structure is formed in a single RIE etch step of a chlorine based chemistry. As shown in FIGS. 1a and 1b, the RIE etch step results in a non-tapered structure with vertical sidewalls.
All of the above prior art methods are directed to forming multi-layer metal structures. However, none address all three of the problems below:
(a) thinning of additional metal layers crossing the edges of the multi-layer metal structure; PA0 (b) shorts or pinholes formed in one or more insulator layers above multi-layer metal structure due to near vertical or undercut edges; and PA0 (c) controlling the effective width of the multi-layer structure when using an extended non-directional overetch.